First of all what does this mean and what are the advantages/disadvantages of both.

Secondly how do I tell which one I have? I have a 2000 but it was built early in the production run and I read in another thread it's possible that I have a high pinion instead of the low pinion that most of the 2000-01s have.

 

Take a look at the back side of your front differential. If the driveshaft input is more toward the top then you have a high pinion. If it's more toward the bottom then you have a low pinion. A high pinion front diff is better for a lifted rig. You can go higher and flex more before the front driveshaft U-joints start to bind.

 

HP axles up front are stronger. the strength is on the drive side of the gears, and its higher up which means better clearance. HP30s are a lot stronger than LP30s, but when you start moving on to larger axles the strength difference becomes less.